1. Field of the Invention
The present invention relates to an induction heating apparatus which is applied used to heat zinc-plated steel plates, for example.
2. Description of the Background Art
Generally, as shown in FIGS. 11, 12 and 13, an induction heating furnace 120 of an induction heating apparatus which heats plated steel plate 119 uses a heating coil 115 of a solenoid type. The plated steel plate 119 which is continuously fed into the induction heating furnace 120 passes through the heating coil 115. A high frequency current is supplied to the heating coil 115 through a conductor 116 from a high frequency power source 117. Thus, as shown in FIG. 14, an induced current (eddy current) 122 is generated in the plated steel plate 119 as a result of the high frequency current 125 to heat the plated steel plate 119 so that an alloy of a plated layer 123 and a steel portion 124 is formed.
The conventional induction heating furnace 120 of the induction heating apparatus of this type, as shown in FIGS. 16 and 17, includes a solenoid coil 115 constituting an induction heating coil which is disposed in a coil support frame 126 and to which a high frequency current (1 to 50 KHz) suitable for a thickness of the steel plate is supplied. The steel plate passes through the solenoid coil 115 so that the steel plate is heated by the Joule heat due to an eddy current generated in the steel plate. In FIG. 17, numeral 127 denotes a heat insulating material.
Further, as shown in FIG. 18, the conventional high frequency power source 117 used in the induction heating furnace 120 includes a high frequency inverter 103 connected to a dc power source 111 having an output connected to a load coil 101 and a condenser 102 which are parallel-resonated, a voltage detection circuit 104 to which an output voltage of the high frequency inverter 103 is supplied, and a trigger pulse generating circuit 109 which supplies signals to each of gates of thyristors 103a, 103b, 103c and 103d constituting the high frequency inverter 103.
In the circuit shown in FIG. 18, the high frequency inverter 103 can be expressed by an equivalent circuit as shown in FIG. 19 in which the thyristors 103a, 103b, 103c and 103d are replaced by switches 113a, 113b, 113c and 113d, respectively, and the dc power source is connected to a load circuit 114. One state in which the switches 113a and 113d are closed and the switches 113b and 113c are opened and the other state in which the switches 113b and 113c are closed and the switches 113a and 113d are opened are alternately repeated to supply ac current to the load circuit 114.
The thyristors 103a, 103b, 103c and 103d can be turned on by external signals. However, since the thyristors can not be turned off by external signals, a state in which the thyristors 103a, 103b, 103c and 103d are simultaneously on is prepared and a reverse current is caused to flow through the pair of thyristors 103a and 103d or 103b and 103c which have been turned on earlier by a condenser 102 connected in parallel with the load coil 101 so that the pair of thyristors 103a and 103d or 103b and 103c are turned off.
In order to generate the state in which the thyristors 103a, 103b, 103c and 103d are on simultaneously, the trigger pulse generating circuit 109 supplies trigger pulses having a phase advanced by .gamma. angle with respect to a load voltage to the gates of the thyristors 103a and 103d or 103b and 103c to turn on the thyristors 103a and 103d or 103b and 103c at the timing advanced by .gamma. angle with respect to the load voltage. As a result the load circuit 114 is supplied with a load current having a phase advanced by .gamma. angle with respect to the load voltage and a frequency corresponding to a resonance frequency of the load circuit 114.